This disclosure relates to a structure of a semiconductor device having a main electrode on a surface of a semiconductor substrate and configured to function as a diode, and a method of manufacturing the same.
Silicon carbide (SiC) is used as a material of a power semiconductor device, and particularly, a SiC Schottky barrier diode is widely used. However, the SiC Schottky barrier diode has a problem that it is may be broken down by a forward surge current. In order to solve the problem, a diode having an MPS (Merged PIN Schottky) structure as disclosed in JP-A-2011-165880 is used, for example. In the MPS structure, a Schottky barrier diode and a pn-junction diode are formed on a surface of a semiconductor substrate having an n-type layer of SiC formed thereon, and a common main electrode to the both diodes is formed on the surface. In the structure, the semiconductor substrate having an n-type layer of SiC formed on the surface is used and p-type regions are discontinusly formed on the surface. According to the structure, in a case where the forward surge current flows, holes are implanted from the p-type regions to the n-type layer, so that a surge tolerance dose is improved.
In the above structure, regarding a material of the electrode formed on the surface, a metal material Schottky-contacting the n-type SiC and ohmic-contacting the p-type SiC is ideal. However, it is difficult to actually obtain the material having the corresponding characteristic. Also, the contact state (Schottky, ohmic) with the semiconductor layer (n-type, p-type) depends on a heat treatment condition (a heat treatment temperature) after the formation of the electrode, and a combination of the material and the heat treatment condition for obtaining a favorable Schottky contact and a combination of the material and the heat treatment condition for obtaining a favorable ohmic contact are different. For this reason, it is very difficult to actually configure the main electrode with a single electrode material.
Therefore, according to the technology disclosed in JP-A-2011-165880, a Ti—Ni alloy is locally formed only on the p-type regions, and then it is subject to a heat treatment at high temperatures of 650° C. or higher to thus form ohmic electrodes. After that, a Mo alloy is formed to cover the ohmic electrodes and the n-type layer surface, which is then subject to a heat treatment at temperatures of 450° C. to 650° C. lower than the above heat treatment to thus form a Schottky electrode. FIG. 5 is a sectional view of a semiconductor device having the corresponding structure. Here, a semiconductor substrate 90 having p-type regions 92 formed on a surface of an n-type semiconductor layer 91 is used, and ohmic electrodes 93 are formed on the p-type regions 92. Then, a Schottky electrode 94 is formed to cover the exposed n-type semiconductor layer 91 and the ohmic electrodes 93. Actually, an Al alloy layer becoming a bonding pad or wiring layer is further formed thereon, as a main electrode (an anode electrode).
According to a method of manufacturing the above semiconductor device, since the process of forming the ohmic electrodes 93 is first performed at the higher heat treatment temperature, it is possible to obtain the ohmic electrodes 93 having a low contact resistance with the p-type regions 92 and the Schottky electrode 94 having a favorable Schottky characteristic with the n-type semiconductor layer 91, respectively. Also, since the ohmic electrodes 93 and the Schottky electrode 94 are made of metal, a contact resistance therebetween is low. Therefore, when the main electrode is formed on the Schottky electrode 94, a diode of an MPS structure having a favorable characteristic is obtained.